In the context of this invention, the term "composite" describes a material consisting essentially of high strength fibers or filaments of graphite, or other material, embedded in a matrix of a thermosetting resin which serves when cured to maintain the alignment of the fibers and their relationship to one another within the matrix as the material is stressed. As applied to the construction of aerodynamic surfaces, composite material has heretofore taken the form of woven mats preimpregnated with resin and, more recently, resin tapes embedded with fibers or filaments aligned in the longitudinal direction of the tape, multiple courses of which are laid side-by-side to construct one ply or layer of a manufactured article which is then constructed incrementally of successive layers of tape.
The application of these construction techniques to contoured surfaces heretofore has been essentially one of first laying up the laminated structure on a flat surface and then transferring or pressing the layup into a mold having the final contour of the part to be fabricated so that the layup will assume the desired shape. The mold with the composite layup applied thereto is then autoclaved. The layers of resinous matrix material merge into a unitary structure during the initial stages of the process and then solidify upon continued exposure to the high temperature in the autoclave as the resin cures.
The described system poses a number of problems in the molding of surfaces of compound curvature i.e., those curved in multiple planes or on multiple axes. One problem is conforming a plane-table layup to the compound surface of a mold and this problem becomes more prominent with the severity of the curvature encountered. In all cases, irrespective of curvature, the mere necessity of transferring the layup from a flat lay surface to a mold and pressing it into conformity with the mold surface is a labor-intensive and time-consuming operation.
It is accordingly desirable from the standpoint of manufacturing efficiency and the integrity of the final product to form the laminated layup with composite tape layed directly upon the compound surface of an appropriately shaped tool or mandrel. This is preferably done with composite tape to make the most efficient use of the strength of the fibrous material, as well as to conform the essentially planar form of the building material more readily to the compound curvature of the mandrel. Such conformance is much more easily accomplished with composite tape than by the use of broad goods.
Even with composite tape, however, the practical necessity of working with tapes of finite widths in the range of from 1 to 6 inches and thicknesses of about 0.0055 to 0.010 inches, and the essential inelasticity of the fiber core of the tape under laying conditions, create their own problems. The primary difficulty is conforming the composite tape to the compound surface without puckering one edge or the other of the tape as the tape laying mechanism follows, within limits, the curvature of the lay surface on any selected tape course.
To overcome this problem, it has been proposed, as disclosed in co-pending application Ser. No. 794,172 by Lewis et al., now U.S. Pat. No. 4,696,707 which is commonly assigned with this invention, that any given lay surface first be defined mathematically with respect to the tape laying machine coordinate system, and that the tape be applied to such a surface by following a preprogrammed natural path of the tape thereon while conforming as nearly as possible to the direction in which the designer would prefer to have the fibers aligned for the sake of the strength of the part. By the technique disclosed in Lewis et al., the path of each successive course within a ply, and each successive ply in the layup of the laminated article, is predetermined such that the machine is programmed to lay the tape without tensioning the tape edges unequally. This avoids the puckering of the tape along either of its edges as would inevitably happen if the laying mechanism sought to steer the tape forcibly to any substantial degree away from its natural path.
To rapidly and accurately generate a natural tape path for a part program, the programmed machine of Lewis et al. uses a mathematical description of the lay surface on the workpiece area of a mandrel forming the complex contoured shape. This mathematical description of the lay surface describes the shape and contours of the mandrel in terms of various Z-axis heights or offsets from a control plane broken into convenient X-Y areas. Because it is the tape laying head of the machine which is to be moved, the X-Y areas of the control plane are referenced not to the actual coordinates in space of the surface of the mandrel but to the internal coordinate system of the tape laying machine. Therefore, there is at least one area of concern in laying tape precisely on the mandrel which must be addressed to enhance the rapid manufacture of parts in this manner.
The mandrel surface may be out of alignment with the machine coordinate system. The part program assumes the actual mandrel surface is oriented to the internal coordinate system of the tape laying machine in the same manner as the mathematical representation. The reason for any discrepancy is that the mandrels used in laying up large parts for the aircraft industry are relatively expansive and bulky, and further are many times assembled in sections. After a part is layed up, the mandrel with the semi-cured composite tape thereon is moved to the autoclave for curing. To not interrupt the production process, another mandrel is brought into place and aligned in the same orientation with the tape laying machine as the other as quickly as possible. However, to manually align an object as large as a surface used for a wing skin to the precision necessary for maximum accuracy of the tape laying machine is extremely time consuming.
A relatively accurate alignment can be made quickly by manual positioning but it is the final adjustment process which requires most of the time. During the time an operator of a tape laying machine is positioning a mandrel to correct the alignment, no tape is being deposited on the mandrel surface and such time is wasted overhead in the production process. What is necessary is a rapid method of aligning relatively large mandrels, or for that matter any large tool or workpiece, to the same coordinate system as the machine working on them so that the process can be run with precision.